From
the horse's mouth
"A personal desktop installation, including a graphical desktop environment, requires at least 1.7GB of free space. Choosing both the GNOME and KDE desktop environments requires at least 1.8GB of free disk space.
A workstation installation, including a graphical desktop environment and software development tools, requires at least 2.1GB of free space. Choosing both the GNOME and KDE desktop environments requires at least 2.2GB of free disk space.
A server installation requires 850MB for a minimal installation without X (the graphical environment), at least 1.5GB of free space if all package groups other than X are installed, and at least 5.0GB to install all packages including the GNOME and KDE desktop environments.
A Custom installation requires 475MB for a minimal installation and at least 5.0GB of free space if every package is selected."
Of course, I think a full installation includes something like 4 Web Browsers, 2 or 3 office/productivity suites, 6 GUI options, 2 Web server apps, 6 text editors, an entire suite of scienticfic tools, an entire suite of programming tools, etc., etc., so you won't really need that much space for a typical install. Do make sure to leave enough breathing room to store your data and programs added post-install though, of course.
By the way- RH 9 is the last "free" version of the official "Red hat Linux"; that line has been spun off into the Fedora Project.